1. Field of the Invention
The present invention relates to the field of active notch filters. More particularly, the present invention relates to a channelized frequency-tunable active notch filter with a low-distortion passband response.
2. Description of the Prior Art
In modern broadband system applications, where receivers are especially vulnerable to signal interference, care must be taken to suppress incidental, unwanted signals that can degrade receiver performance through the generation of nonlinearity-induced spurious signals. A conventional broadband power limiter, often implemented with semiconductor p-i-n diodes, can provide effective protection against receiver front-end burnout, but will cause spurious responses of its own, due to the nonlinear action of the limiting process. An alternative is to place a notch band-reject filter in front of a receiver""s low-noise amplifier to notch out potential interference, with frequency-tunability added to permit adaptation to changing incidental signal conditions. This type of filter will subsequently be referred to as a tunable notch filter, for short. Because of the strategic up-front position of the filter, it is essential that the passband-determining portions of the circuit be as free as possible of losses and nonlinearities to minimize noise contributions and signal distortion, respectively. Although active filters have the ability to provide good frequency selectivity, as well as reduce circuit size, weight, and cost, which are the critical requirements in modern, multifunction, wide-bandwidth system applications, they can be at a disadvantage when it comes to noise, susceptibility to signal distortion, and signal-level dependence of transfer characteristics.
A common way to realize an active notch filter is to augment a passive notch filter structure with active loss-compensation circuitry that involves regenerative feedback, yielding overall designs that display good frequency selectivity, yet are small in size. Among the particular disadvantages of this approach are high noise levels, and the potential for parasitic oscillations, should the active element gain change with temperature or age.
Active notch filters, in the past, have exhibited very high noise figures and high signal distortion levels, even at passband frequencies where no filtering action takes place. This is due to the presence of impedance matching networks in the main signal path that introduce noise, and the presence of active circuit elements in the form of transistors and amplifiers that contribute both noise and signal distortion.
An attractive alternative, which does not harbor the potential for parasitic circuit instabilities and possesses other distinguishing attributes, is to employ a channelized configuration that relies on interaction among channel feed-forward signal components to generate a sharp rejection notch, as described in U.S. Pat. No. 5,339,057 to Rauscher and an article entitled xe2x80x9cMicrowave Channelized Active Filtersxe2x80x94A New Modular Approach to Achieving Compactness and High Selectivityxe2x80x9d authored by Rauscher in the IEEE Microwave Theory Tech. Transactions, vol. 44, pp. 122-132, Jan. 1996.
The simplest implementation of a channelized notch filter encompasses two parallel-connected channels, comprising one frequency-selective channel with a narrow bandpass response, and one bypass channel with an associated phase response that establishes a direct signal path from the filter input to output. A representative example of such a filter, in block-diagram form, is shown in FIG. 1. The frequency-selective channel, with its left-hand-side and right-hand-side bandpass filters BPF11 and BPF12, invariably requires a unilateral amplifier, G11, to compensate for passive-circuit losses as well as to direct signal flow. The bypass channel, whose main purpose is to provide phase shift with the help of left-hand-side and right-hand-side phase-shift networks PSN11 and PSN12, on the other hand, often requires another unilateral amplifier G12 solely to assure feed-forward signal propagation in the channel, as associated passive-circuit dissipation losses are generally low, due to the simplicity and the non-resonant nature of the structures typically involved. With the composite notch filter""s passband response determined primarily by the characteristics of the bypass channel, any amplifier used therein is apt to be a prominent source of passband noise and distortion.
In the bypass channel of a two-branch channelized notch filter, the use of a unilateral gain block or amplifier is unavoidable, due to stability and other considerations.
Therefore, it is an objective of certain embodiments of the present invention to provide a circuit architecture that preserves the unilateral characteristics of the bypass channel without requiring the actual presence of a gain block or amplifier in that channel.
It is another objective of certain embodiments of the present invention to provide a frequency-tuned active notch filter, which adopts a circuit architecture in which the passband determining parts of the filter are essentially free of noise- and distortion-generating active circuit elements, and also largely free of lossy impedance-matching networks that can add to passband noise.
It is still another objective of certain embodiments of the present invention to provide a single-pole bandpass filter that can accomplish frequency tuning using only one variable capacitance element, yet still maintains constant passband width across the tuning span.
Therefore, it is still a further objective of certain embodiments of the present invention to provide a notch filter that combines the low-distortion qualities of passive notch filters with the performance, size, weight, and cost qualities of active notch filters.
In a first aspect, the present invention relates to a tunable notch filter which comprises a frequency-selective four-port directional coupling network and a reciprocal, passive phase-shift network. Upon receiving an input signal within a predetermined frequency band, the frequency-selective four-port directional coupling network directs a part of the input signal received at the notch filter input port to the reciprocal, passive phase-shift network before directing it further to the filter output port. The coupling network directs the other part of the input signal directly to the output port of the notch filter. The part of the signal passing through the phase-shift network and the part of the signal passing directly to the filter output port may cancel each other at the filter output port. Therefore, the input signal within the predetermined band is filtered out by the notch filter. Upon receiving an input signal outside the predetermined frequency band, the frequency-selective four-port directional coupling network directs the entire input signal to the reciprocal, passive phase-shift network before directing it further to the output port of the notch filter. Therefore, the input signal outside the predetermined band propagates substantially unperturbed from the filter""s input port through the reciprocal phase-shift network to filter""s output port.
In a second aspect, the present invention relates to a channelized notch filter which comprises three parallel channels. The first channel comprises a first bandpass filter, a first phase-shift network, and a second bandpass filter. The second channel includes a third bandpass filter. The first and the second channels may be interchanged. The third channel includes a second phase-shift network. The input ports of the three channels are all electrically connected to the input port of the notch filter. The output port of the notch filter is connected to the output port of either the first channel or the second channel. The output ports of the first and the second channels are electrically connected to one another through a third phase-shift network. The output port of the third channel is directly connected to the output port of one of the other two channels, which is not connected directly with the filter output port.
The notch filter of the present invention offers, for the first time, a microwave active notch filter approach that significantly reduces passband noise, when compared to earlier techniques, and virtually eliminates filter-generated passband signal distortion. The notch filter of the present invention may still possess most of the desirable characteristics of a microwave active notch filter with regard to combining good frequency selectivity with small size and weight, and low potential cost. In addition, the notch filter of the present invention may retain most of the desirable attributes of prior-art channelized active notch filters, including unconditional circuit stability, modular circuit assemblies, and exceptionally high tolerance for passive-element dissipation losses, thereby permitting the use of compact, but lossy lumped circuit elements within the context of a planar microwave-monolithic-integrated-circuit format.
In a third aspect, the present invention relates to a bandpass filter that tunes in frequency by tuning only one element with a variable or tunable reactance, yet still maintains a constant passband width across the tuning span. An important feature of the bandpass filter of the present invention is that it permits the achievement of constant notch bandwidth across the entire frequency-tuning span of a notch filter in which the bandpass filter is incorporated as a component. The solution demonstrated here involves bandpass filters with capacitively end-coupled resonators, where each resonator is split in half at the electrical symmetry point. This provides optimum impedance-transformation properties that render the bandpass channel""s passband width, and hence the notch filter""s notch bandwidth, largely invariant to tuning frequency.